Variable inductance



'March 11, 194:7. R, Q CLARK VARIABLE INDUCTANCE Filed July 15, 1944Pateiied Mar. 11, 1941 VARIABLE INDUCTANCE Robert George Clark, London,England, assigner to The Hartford National Bank and Trust Company,Hartford, Conn., trustee Application Jiuy 13, 1944, serial No. 544,791In Great Britain September 23, 1942 This invention relates to variableinductances ofthe kindcomprising two coils usually connected in seriesand movable in relation to each other to effect the variation. Theinvention is concerned more particularly with variable inductances ofthis kind in which the coils are constructed and mounted so as to slideone within the other, e. g. are wound on cylindrical formers ofdifferent diametersand are arranged coaxially.

The object of the present invention isto provide an improved variableinductance comprising axially aligned and relatively movable sections,wherein the range of variation in inductance which can be achieved issimilar to that normally achieved by the use of a fixed inductance andvariablcondenser; the relation between lowest and highest frequency isusually roughly 1:3 and if tuning is to be effected by a variableinductance the corresponding ratio between the inductance values is 1:9which is far greater than can be attained with a two part coil havingits parts arranged co-axially with one part sliding within the other.adapted for ganglng and afford a simple arrangement for tuning inwireless receiving apparatus.

According t the invention, with this object in view, the inductanceconsists of two co-axially mounted parts connected in series and in suchmanner as to provide vpositive mutual inductance between them andadapted to slide onewithin the other, and at least one of the two partsis rigidly connected with a co-axial auxiliary winding which when thetwo parts are one Within the other in their position of maximum couplingis substantially without effect but when the two parts are moved apartthe auxiliary winding rigidly connected with one of the parts assumes aposition of maximum coupling in relation' to the other part and reducesthe total inductance.

In order that the invention may be more readily understood, it will nowbe described with reference to the accompanying drawing in which twoembodiments of the invention are shown and in which:

Fig. l is a schematic diagram illustrating a coaxal coil arrangement forobtaining maximum coupling between two coils;

Fig. 2 is a schematic diagram illustrating a similar coaxial coilarrangement with minimum coupling between the two coils;

Fig. 3 is anl exploded longitudinal sectional view of a coaxial coilarrangement for varying the coupling between the two coils;

Such variable lnductances are well- 8 Claims. (Cl. 171-4-119) Fig. 4 isa longitudinal sectional view of a coaxial coil according to theinvention for varying the ycoupling between the two coils;

Fig. 5 is a schematic illustration of a second embodiment of theinvention in which the coils are arranged for minimum coupling;

Fig. 6 is a schematic illustration of a second embodiment of theinvention in which the coils are arranged for maximum coupling;

Fig. 7 is a sectional view of the second embodiment of the invention forvarying the coupling between a pair of coaxial coils;

Fig. 8 is a schematic circuit arrangement showing the parallelconnection ofthe coils illustrated in Fig. 5; and

Fig. 9 is a schematic circuit arrangement showing the series connectionof the coils illustrated in Fig. 5.

The principle on which the embodiment of Figs. 3 and 4 is based, willfirst be explained with reference to Figs. 1 and 2. Here, L1 and L2 arethe two parts of the inductance which are mounted coaxially and areconnected in series in such manner as to produce positive mutualinductance between them. Associated with the part L1' is a shortcircuited winding S1 and associated with I the part L2 is a shortcircuited winding Sz, the 1 i disposition of the components being suchthat when L1 and L2 are one within the other in they." position ofmaximum coupling (Fig. l) the short j circuited windings S1 and Sz areon opposite sides of the coupled parts L1 and L2 and are substan- 1 Onrelative displacement tially Without effect. into the position ofminimum coupling (Fig. 2) each of the parts L1 ,and L2 is closelycoupled with the short circuited winding (S2 and S1) associated with theother, whereby coupling between L1 and L2 is substantially eliminated.The position of maximum coupling (Fig. l) between L1 and Le affords themaximum inductance value whereas the minimum inductance value isobtained when the positions are such that the short circuit windingshave the maximum coupling with the inductances. Intermediate settingsbetween Figs. l and 2 give corresponding intermediate inductance values.The short circuit wind-L ings Siv and S2 are most convenientlyconstituted by metal cylinders or collars, preferably of copper.

In the embodiment shown in Figs. 3 and 4 the part L1 of the variableinductance is wound on a former F1 to one end of which .is secured aplate P serving to mount the variable inductance. Also secured to theplate Pand extending into the former F1 is a copper cylinder S1. Thecylinder S1 and the plate P are provided with a central aperture toreceive the rod R on which the other part of the inductance is secured.The copper cylinder S1 constitutes the short circuited windingassociated with L1 and the gap between S1 and the former F1 on which L1is wound is so dimensioned as to enable the former F2, on which theother part L2 of the inductance is wound, to pass into the gap. Theformer F2 is secured to a copper cylinder S2 which is co-axial with andrigidly secured to the rod R. This cylinder S2 constitutes theshort-circuited winding associated with La. The axial length of S1should be not less than the axial length of Le, and similarly for Se andL1. Fig. 3 shows the two parts L1 and L: widely separated in order thatthe details of construction should be clearly seen. Fig. 4 shows anactual working position in which the total value of the inductance is a.maximum. For smaller values, the rod R is moved to the right togetherwith the copper cylinder S2, the former F2 ,and the part Le of theinductance. With this construction an induction ratio of at least :1 anda Q of approximately 50 can readily be attained. By disposing both S1and S1 so as to be within the coils L2 and L1 with which they co-operateas shown in Figs. 3 and 4 it is found possible to achieve morefavourable Q values than by arranging one inside and the other outsideas in the explanatory diagrams of Figs. l and 2.

In the embodiment of Figs. 3 and 4, and in general for constructionsbased on' the principle of Figs. land 2, itis preferred that the partsL1 and L2 of the inductance should be of the same axial length and alsohave the same inductance value.

Referring now to the explanatory diagrams of Figs. 5 and 6, L1 and Lahave the same signicance as before. However the coil La has associatedwith it a supplementary coaxial coil In which is secured in fixedrelation to the coil Ln and is electrically connected so as to producenegative mutual inductance effects with L1. The coils La and La arepreferably spaced axially by a distance equal to the axial length of L,and the arrangement is such that by relative axial movement between L1on the one hand and La and La on the other hand, L1 may be disposedformaximum coupling with La or with La or in any intermediate position.Fig. 5 shows the minimum total inductance position and Fig. 6 shows thesetting for maximum total inductance.

Fig.- 'l shows a construction based on Figs. 5 and 6. \In Fig. '1, thepart L1 of the inductance is wound on a formerF1, whereas the part Leand the supplementary coil Le are wound on a common former Fa which isof external diameter to fit within the former F1. By relativeaxial-move- 'ment of F1 and Fa the part L1 -of the inductance can bebrought into any desired position in relation to La and le, between aminimum total inductance setting equivalent to Fig. 5 and a maximumtotal inductance setting equivalent to Fig. 6.

Advantageously the lcoils L1, Lz and 1c all have the same axial lengthas shown in Fig. 7.

The coils L2 and La may be connected either in series or in parallel,but in either case the combination of Le and L1. Fig. 8 shows thecircuit arrangement when L2 and Le are in parallel, and Fig. 9 shows aoircuit arrangement whenL1, Ln and'Ls are all in series. With a circuitas in Fig. 8 it is preferred that Le and La should have the sameinductance value. numerically equal to twice the value of L1. With acircuit as in Fig. 9 it is preferred that La is connected in series with4 L1. La and Le should all have equal inductance values.

I claim:

1. A variable inductance comprising two coaxially mounted partsconnected in series so' as to provide positive mutual inductance betweenthem and adapted to slide one within the other, at least one of saidparts being rigidly connected with a coaxial short-circuited windingarranged to be substantially without effect when the two parts are onewithin the other in their position of maximum coupling, but to assumewhen the two parts aremoved apart, a position of maximum coupling inrelation to the other part whereby to reduce the total inductance.

2. A variable inductance comprising two coaxially mounted partsconnected in series so as to provide positive mutual inductance betweenythem and adapted to slide one within the other, at least one of saidparts being rigidly connected with a coaxialshort-circuited windingarranged to be substantially without effect when the two lparts are onewithin the other in their position of maximum coupling, but to assumewhen the two parts are moved apart, a position of maximum coupling inrelation to the other part whereby to reduce the totall inductance, saidshort-circuited winding being provided in the form of a metal collar.

3. A variable inductance'comprising two coaxially mounted partsconnected in series so as to provide positive mutual inductance betweenthem and adapted to slide one within the other, each of said parts beingrigidly connected with a coaxial short-circuited winding arranged to besubstantially without effect when the two parts are one within the otherin their position of maximum coupling, but to assume when the two partsare moved apart, a position of maximum coupling in relation to the otherpart whereby to reduce the total inductance.

4. A variable inductance comprising two coaxially mounted partsconnected in series so as to provide positive mutual inductance betweenthem and adapted to slide one within the other,

each' of said parts being rigidly connected with a coaxialshort-circuited winding arranged to be substantially without effect whenthe two parts are one within the other in their position cf maximum.coupling, but to assume when the two parts are moved apart, a positionof maximum coupling in relation to the other part whereby to reduce thetotal inductance, said short-circuited winding' being provided in theform of a metal collar.

5. A variable inductance comprising two coaxially mounted partsconnected in series so as to provide positive mutual inductance betweenthem and adapted to slide one within the other. each o! said parts beingrigidly connected with a coaxial short-circuited winding arranged to besubstantially without eiect when the two parts are one within the otherin theirv position of maximum coupling, but to assume when the two partsare moved apart, a position of maximum coupling in relation to the otherpart whereby to reduce the total inductance, the short-circuited windingassociated with the outer part being mounted within it at such spacingas to permit the passage of the inner part between the outer part andthe short-circuited winding associated therewith.

6. A variable inductance comprising two coaxially mounted partsconnectedin series so as to provide positive mutual inductance between tnem andadapted to slide one within the other, at least one of said parts beingrigidly connected with a coaxial short-circuited winding arranged to besubstantially without effect when the two parts are one within the otherin their position of maximum coupling, but to assume when the two partsare moved apart, a position of maximum coupling in relation to the otherpart whereby to reduce the total induetance, the axial lengths of thetwo parts of the inductance and of the short-circuited winding beingmutually equal.

7. A variable inductance comprising two coaxially mounted partsconnected in series so as to provide positive mutual inductance betweenthem and adapted to slide one within the other, each oi' said partsbeing rigidly connected with a coaxial short-circuited winding arrangedto be substantially without eiect when the two parts are one within theother in their position of maximum coupling, but to assume when the twoparts are moved apart, a position of maximum coupling in relation to theother part whereby to reduce the total inductance, the axial lengths ofthe two parts of the inductance and of the short-circuited windingsassociated therewith being mutually equal.

8. A variable inductance comprising two tubular parts slidably arrangedone within the other for longitudinal displacement relative to oneanother, series-connected coils mounted on each oi said parts andadapted to provide positive mutual inductance between them, a cylinderconstituting one short-circuited winding connected to and arrangedwithin the outer part spaced from the interior wall thereof, and asecond cylinder constituting another short-circuited winding connectedto and mounted on the inner part, the tubular portion of said inner partbeing adapted to slide into the space between the outer part and saidrst-named cylinder whereby each cylinder is positioned within the partof the inductance with which it cooperates.

ROBERT GEORGE CLARK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,441,532 Fortin Jan. 9, 19232,334,178 Dodge Nov. 16, 1943 FOREIGN PATENTS Number Country Date388,571 British Mar. 2, 1933

